The present invention relates to color mixing, in particular to color-image formation in an optical fiber laser system for use in display units, virtual displays, laser printing, laser TV, laser lithography, scanning, etc.
For better understanding the principle of the invention, it would be advantageous to briefly describe some fundamentals of color mixing in an image-formation process. It is well known that perception of colors by a human eye is a complicated physiological process, which involves participation of an optical system of an eye and our brain. In other words, color is the external factor, a sensation existing only in our minds.
The remarkable repertoire of color effects seen by the eye, and interpreted by the brain, is our response to certain wavelengths of electromagnetic energy that makes up the visible spectrum of light. While we are able to measure quite accurately the wavelengths of light, our response to those wavelengths is affected by a whole multitude of factors that are beyond the scope of our invention. What is important to note is that, when we observe a non-self-luminescent object, i.e., an object illuminated by an external light source, the color perception of an eye from the light reflected from this object will depend essentially only on the spectral composition of the light that illuminates this object. Thus, a color is a property of spectral characteristics of light radiation common for the entire radiation spectrum, irrespective of visible or invisible. Generally speaking, light can be expressed as a combination of three interconnected parameters, i.e., by colors of three-color components, which are known as base colors. Each of these base colors can be the one that cannot be reproduced by mixing two others. There are a great variety of three basic colors that can reproduce the entire range of the color spectrum. Among those, three basic colors, i.e., red (R), green (G), and blue (B) form the most popular system known as an RGB system. By mixing the RGB colors we can produce practically any color shade of a visible color. The principle of RGB color mixing system find wide practical application in color TV, flat panel displays, color printing, etc.
There are known various color definition and classification systems. These systems define various colors and standardize them. In 1931, the International Commission on Illumination has standardized the following wavelengths for the RGB color mixing system: 700 nm for R, 546.1 for G, and 435.8 for B. Therefore development of new effective color mixing systems still remains an actual problem of the technology.
It is important to notice that, in the aforementioned standards, unit quantities of basic colors are selected so that a ratio of their energetic brightnesses is equal to a predetermined value, i.e., 72.1:1.4:1.0. This ratio, however, will be changed, if we chose, e.g., 653 nm for red, 530 nm for green, and 458 nm for blue.
Heretofore color-mixing systems with electronic control have been known. One of the latest developments in this field is a color image formation system described in U.S. Pat. No. 5,832,155 issued in 1998 to Rasch, et al. This invention concerns an integrated-optical junction splitter, in particular for applications in the wavelength range of visible light, which ensures a spatial and wideband combination of light in a wavelength spectrum xcex94xcex greater than 75 nm (value given applies to short-wave visible light). The system uses a single white light source or several light sources with a wide wavelength distance. In the case of a usable wavelength range comprising the entire spectrum of visible light, the junction splitter is a white light junction splitter. The aforementioned junction splitter consists of at least three channel waveguides, at least one of which must be a single-mode integrated-optical wideband channel waveguide (SOWCW). Two channel waveguides, with each having a respective input, are combined into a common SOWCW at their outputs in a coupling point, which common SOWCW features a common light output at its end. This wideband junction splitter is used as a wavelength-selective or wavelength-independent switch or modulator, in interferometric and photometric devices, sensors, and microsystem-technical solutions.
The aforementioned U.S. Pat. No. 5,832,155 describes only passive optical waveguide splitting and mixing of various wavelengths in a solid-state waveguide embedded in a solid-state material, such as LiNbO3. No electronic control is mentioned.
The system described in U.S. Pat. No. 5,832,155 has the following disadvantages. The white light junction splitters used in the system are optically and electrically inefficient because only few microwatts of light energy (less than 1% of the entire light energy emitted from the source) can be coupled from the white source. This inefficiency generates heat-removal problems, since the unused energy is converted into heat inside the device. For the same reason, electrical consumption of the system of the aforementioned patent is not suitable for portable devices since a high-capacity power source is needed.
Furthermore, embedding of titanium waveguides into the matrix of TiNbO3, as well as processing of electrodes used in the aforementioned patent, are expensive and complex processes that include photolithography and other complicated operations.
The only light source for all spectral (wavelength) components of the system is a white light source that must be always ON for operation of the system. This means that when modulation signals, i.e., high-voltage pulse signals U1, U2, U3, are sent to the electrodes for adjusting output light powers of individual waveguides, the process consists in xe2x80x9cclosingxe2x80x9d unnecessary waveband portions while passing to the output the selected portions. This means that a major part of light energy is lost, and the system operates with low efficiency.
U.S. Pat. No. 5,802,222 issued in 1998 to the same applicants as U.S. Pat. No. 5,832,155 discloses a color image generation system for the reproduction of real or virtual, two-dimensional or three-dimensional, color, or monochrome image, in particular for television, video, or printing applications. Integrated-optical structures are used on a mount, in particular a substrate. An image is generated by deflecting color light beams generated into a viewing space, which deflection is effected in synchronism with color setting and intensity- or amplitude modulation of the light. The patent does not describe but only shows the electronic control assembly and describes in more detail the color image generation system that may be integrated on a mount either monolithically or in hybrid fashion. The color generation system may be implemented as an encased module.
Similar to U.S. Pat. No. 5,832,155, the system of U.S. Pat. No. 5,802,222 is based on adjusting the light intensity signals of at least two wavelengths which are to be mixed so that the final light intensity signal ratio on the output of the color mixer is obtained in accordance with the value required for a specific color. To achieve this purpose, the inventors use various control means such as amplitude attenuators, amplitude modulators and modulators of other types, pulse duration adjusters, etc. Although the description of the electronic control unit is essential for understanding the operation of the system, no disclosure of electronic control is contained in the description. Nevertheless, it can be assumed that for obtaining a great variety of color shades, such an electronic system, would be very complicated and expensive, since it is based on amplitude modulation of component light power signals prior to their mixing.
Known are various systems for electronic control of intensity of light emitted from a laser diode. Thus, U.S. Pat. No. 5,715,021 issued to Gibeau at al. describes a method and apparatus for a multi-application, laser-array-based image system, which utilizes three linear laser arrays. Each linear array generates multiple (N greater than 1) parallel output beamlets at one of the three primary colors (red, green, blue). The corresponding 1 to N output beamlets of the three linear arrays, each individually modulated in luminance according to a specific encoding scheme representing the video image to be produced on the viewing screen, are combined spatially to form a single white light linear array source. Through a projection/scanner optical system, the N output beamlets of the white light source are simultaneously directed to, and swept horizontally across a distant viewing screen, resulting in a swath of N lines of a graphic video image. By producing M contiguous swaths vertically down the viewing screen, a full image of Mxc3x97N lines is produced. The red, green, and blue linear laser arrays may consist of arrays of semiconductor laser diodes made of suitable semiconductor materials so as to directly emit radiation at wavelengths corresponding to red, green, and blue colors, respectively. Alternatively, the red, green and blue color arrays may be formed by arrays of semiconductor laser diodes emitting radiation at twice the desired red, green and blue wavelengths, whose radiation is coupled to arrays of second harmonic generation crystal elements.
The device described in the aforementioned patent is complicated in construction since it relates to three color laser arrays, i.e., to arrays of red, green, and blue colors. Furthermore, some components of the optoelectronic circuits used in the system of U.S. Pat. No. 5,715,021, such as the array of blue color, which corresponds to the basis blue of the standard RGB system, are not yet commercially produced. The optical system for collimation and delivery of light beamlets is also extremely complicated and requires the use of special couplings and optical elements, which also are not yet industrially produced.
It is an object of the present invention to provide an apparatus for color mixing in a laser diode system which has miniature dimensions and therefore is suitable for use in portable devices, possesses high light power efficiency, is free of heat-generation problems, has low electrical consumption, has simple construction and is inexpensive to manufacture, ensures modulation of wavelength signals directly on a laser-diode light source, and ensures mixing of a great number of different colors and color shades with adjustment of output color tones. Another object is to provide a new method of color mixing in a laser diode optical system for obtaining a great variety of colors and color. Another object of the invention is to provide aforementioned apparatus and method, which, in view of their versatility, are suitable for application in different fields from mere color mixing to high-definition TV sets.